The McDonald Observatory search for pulsating sdA stars

Asteroseismic support for multiple populations^⋆

¹ Max-Planck-Institut für Sonnensystemforschung (MPS), Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
e-mail: bell@mps.mpg.de
² Department of Physics and Astronomy, Stellar Astrophysics Centre, Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark
³ Instituto de Física, Universidade Federal do Rio Grande do Sul, 91501-970 Porto-Alegre, RS, Brazil
⁴ Smithsonian Astrophysical Observatory, Cambridge, MA 02138, USA
⁵ Department of Astronomy, University of Texas at Austin, Austin, TX 78712, USA
⁶ McDonald Observatory, Fort Davis, TX 79734, USA
⁷ Baylor University, Waco, TX 76798, USA
⁸ Department of Physics and Astronomy, University of North Carolina, Chapel Hill, NC 27599, USA
⁹ Institut für Theoretische Physik und Astrophysik, Universität Kiel, 24098 Kiel, Germany

Received: 22 April 2018
Accepted: 26 May 2018

Abstract

Context. The nature of the recently identified “sdA” spectroscopic class of stars is not well understood. The thousands of known sdAs have H-dominated spectra, spectroscopic surface gravity values between main sequence stars and isolated white dwarfs, and effective temperatures below the lower limit for He-burning subdwarfs. Most are likely products of binary stellar evolution, whether extremely low-mass white dwarfs and their precursors or blue stragglers in the halo.

Aims. Stellar eigenfrequencies revealed through time series photometry of pulsating stars sensitively probe stellar structural properties. The properties of pulsations exhibited by sdA stars would contribute substantially to our developing understanding of this class.

Methods. We extend our photometric campaign to discover pulsating extremely low-mass white dwarfs from the McDonald Observatory to target sdA stars classified from SDSS spectra. We also obtain follow-up time series spectroscopy to search for binary signatures from four new pulsators.

Results. Out of 23 sdA stars observed, we clearly detect stellar pulsations in 7. Dominant pulsation periods range from 4.6 min to 12.3 h, with most on timescales of approximately one hour. We argue specific classifications for some of the new variables, identifying both compact and likely main sequence dwarf pulsators, along with a candidate low-mass RR Lyrae star.

Conclusions. With dominant pulsation periods spanning orders of magnitude, the pulsational evidence supports the emerging narrative that the sdA class consists of multiple stellar populations. Since multiple types of sdA exhibit stellar pulsations, follow-up asteroseismic analysis can be used to probe the precise evolutionary natures and stellar structures of these individual subpopulations.